In the study, researchers separated hamsters into two groups: one group experienced a standard light/dark cycle, while the other group was exposed to a dim light at night--on par with city lights outside your window or the glow of a TV screen. After four weeks, hamsters who slept with some light showed signs of depression. (Depressed hamsters? Yup. Researchers tell by observing the rodents' interest in sugar water.)

The reason? The sleep hormone melatonin helps brain neurons function properly. "But the brain can't produce melatonin if it detects light," says study author Tracy Bedrosian, a Ph.D. candidate in neuroscience at The Ohio State University. Hamsters exposed to light at night had less complex neurons in areas of the brain, such as the hippocampus, which is associated with mood. (Diagnose any major sleep problem at the brand-new Men's Health Sleep Center.)

Human studies are needed to confirm the findings, but there's enough research connecting nighttime light with disease that it's a good idea to keep light dim when the sun goes down, Bedrosian says. Her advice: Install black-out curtains if there's light outside your bedroom window and turn off your computer monitor and TV screen when you go to sleep.